"Muffler or silencer. Any device for silencing, muffling, or diminishing the report of a portable firearm, including any combination of parts, designed or redesigned, and intended for the use in assembling or fabricating a firearm silencer or firearm muffler, and any part intended only for use in such assembly or fabrication"

Note there is no standard for actual reduction of sound, only that the device, or any part thereof, is designed or intended to reduce the sound. There was a case where a gang banger was threatening a person and stuck a potato on the end of his bbl. They charged him with possession of a silencer even though the idea was ludicrous. The court ruled that the gangbanger wasn't intent on finding a quick way to make hashbrowns, the defendant obviously intended for the spud to reduce the sound of the shot and was therefore a silencer even though it might have actually loudened the shot by blowing up the gun.

A single baffle, a threaded end cap or tube are all parts that by themselves can get you convicted if they can convince a jury your intention was to make a silencer out of it. There is even a little plastic adapter out there that adapts the threads on a 2 liter bottle to attach to the end of a bbl. Yep, it is a silencer, and every 2 liter bottle in your possession at the time you are caught with it is an add on charge.

Which are sold without tax to anybody. The stated purpose is to reduce sound, so why isn't it a silencer? Ditto for the .22 ammo like the Colibri that is designed for "quiet pest removal" and such. During Vietnam a 12 guage shell that had a captured piston was developed, when fired the shot was expelled by the piston but the explosion was contained within the shell, no noise. Each shell is a silencer according to ATF. In ATF's entire history, I don't think anybody has ever accused them of being consistent....